All the best GMs I've seen have one thing in common; they like the setting and are well-versed in it. Nothing's worse than a GM who can't adapt to their players going even slightly off-script, or who runs a game that's not internally consistent, because they can't be bothered to give a crap about which political faction that one NPC assassin actually worked for.
I think knowing the world enough that you can improvise things or react well to player initiative is a key part to being a good DM.
And have some enthusiasm for the setting. If someone doesn't give a crap about L5R, then maybe they shouldn't run it, no matter how many of their players are crying about not being able to play Crane Clan courtiers in Shadowrun.
A really half-ass game that takes place in a really awesome setting is the worst kind of game. Expectations are too high, and if the DM can't deliver or doesn't care to put in the work, then it's better to just pull the plug on it before everyone's fun gets ruined.
But a really kick-ass game that takes place in a setting that maybe the players don't like as much, but that the DM can get excited about, will usually turn into something really fun, because it's the DM's job to keep things moving.